In online search, users often have a need that can be met through a number of different web pages. The information or commercial need of the user can often be described by a concept or a set of concepts with a set of constraints. However, most search engines require users to try finding information by knowing what phrases might frequently appear on the page.
Similarly, in online advertising, advertisers have to provide long lists of keywords or phrases that they hope the user would type as part of the query when searching for their products. Even with the advertisers' efforts, some keywords are unique and hard to use for ad targeting. For example, a product SKU can be forgotten when generating the keyword list. Further, the product SKU may be hard to use for searching.